In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an electrostatic latent image formed on a surface of a photoreceptor drum by electrically charging the photoreceptor surface and exposing the photoreceptor surface to light is developed into a toner image with a toner, and a developing roller is used for the development.
More specifically, the developing roller is rotated with a quantity regulating blade (charging blade) in contact with the developing roller, whereby the toner is electrically charged to be applied to an outer peripheral surface of the developing roller, and the toner application amount is regulated by the quantity regulating blade. Thus, a toner layer is formed to a generally constant thickness on the outer peripheral surface of the developing roller.
In this state, the developing roller is further rotated and, when the toner layer is transported to near the surface of the photoreceptor drum, the toner selectively moves from the toner layer to the surface of the photoreceptor drum according to the electrostatic latent image formed on the surface of the photoreceptor drum. Thus, the electrostatic latent image is developed into the toner image.
There is a trend toward the use of a toner including more uniform, more spherical and smaller size toner particles or a polymeric toner. In order to impart such toner with higher electrical chargeability and efficiently develop the electrostatic latent image into the toner image, it is effective to use, as the developing roller, a semiconductive roller having a roller resistance controlled, for example, at not greater than 108Ω.
To meet various requirements imposed on the semiconductive roller, the surface geometry of the outer peripheral surface of the semiconductive roller to be kept in contact with the toner is defined, for example, by a ten-point average roughness Rz, an arithmetic average roughness Ra and the like, or by an area ratio of concavities present in the outer peripheral surface (Patent Documents 1 to 4).